The present invention relates to digital systems of the type used in loop communications systems and more particularly to a protocol circuit for a local area network (LAN) of the token ring type.
A token ring local area network (LAN) is a data communications system which transfers data among a number of stations coupled to a one-way signal path. Each station has a host processor with memory and various periperal devices to function as an independent workstation. Access to the ring is controlled by token passing wherein a free or busy token is passed from one station to the next around the ring. An early example of the token ring is shown in U.S. Pat. No. 3,597,549 issued to Farmer & Newhall on Aug. 3, 1971 and assigned to Bell Telephone Laboratories. The Farmer & Newhall network discloses use of a supervisory station for resolving technical and administrative problems of communication on that loop such as to break in on the transmission of a station which is dominating the loop and to issue a signal that results in all stations losing loop control. Although Farmer & Newhall allows a mode in which control is passed from station to station within a loop without intervention by the supervisory station this occurs only as long as there are no more messages to be transmitted or until trouble develops in the loop. Otherwise, the loop is under control of the supervisory station.
In a further refinement IBM developed the so-called Munich ring which was reported at Proceedings of the National Telecommunications Conference, Nov. 1981, pp. A.2.2.1-A.2.2.6. Standard protocols for these communications methods have been published by the IEEE as the 802.5 token ring proposal, and a similar report by the European Computer Manufacturers organization.
The present invention when used in a local area network system is concerned with the protocol handler for a token ring local area type of network system. There are several possible ways to design a protocol handler circuit. Regardless of the particulars of the design, however, all must carry out four major functions. First, passing frames must be examined, their status recorded and, if necessary, marked or deleted by a monitor. Second the protocol handling circuit must recognize frames and copy those with appropriate addresses, signal the return of transmitted frames to a transmit state machine and signal successful copy and/or address recognition by setting flag bits at the end of the frame. Third the circuit must be able to transmit frames in various modes and for the token transmission mode it must be capable of examining and recognizing incoming tokens to determine whether or not to use them. It must be able to release a token upon return of its frame or on request. Fourth, a proper priority token protocol must be activated by the release of a token from the transmit machine and then remain active as long as necessary as determined by a priority token algorithm. While active it must examine passing tokens and modify them as necessary according to a priority token algorithm. As this involves copying a field from the end to the start of a byte sufficient delay must be allowed in the data path for this translation to be accomplished. It is possible to carry out each of the foregoing functions independently in sequence. However, such an approach would result in cascading of the multiplexers, multiple delimiter decoding to detect the start and end of a token for frame and multiple data modification points. The foregoing would necessarily result in increased complexity, delay and power consumption.
Accordingly, it is a principal object of the invention to provide a protocol handler design that is as simple as possible but yet capable of carrying out the functions mentioned above. It is a further object of the present invention to provide a protocol handler circuit design that introduces a minimum of delay in the ring.